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12                      TRITUBERCULY AND SEXTUBERCULY                    59
lacks finality as a convincing proof of the tritubercular tooth as
a primitive Ungulate tooth.
Professor Osborn has ingeniously utilised certain deviations
from the normal type of tooth structure (for the group) in favour
of his strongly-urged opinions. If the stages of development
have been as he suggests, a retrogression would naturally be in
the inverse order; thus the " apparently ' triconodont7 lower
molar of Thylacinus '* may be interpreted as a retrogression from
a tritubercular tooth. In the same way may be explained the
triconodont teeth of Seals and of the Cetacean ZetLglodon. Finally,
the modern Toothed Whales have retrograded into " haplodonty."
Embryological evidence has also been called in, and with
some success, to contribute towards the proof of the tritubercular
theory of teeth. Taeker has shown that in the Horse and the
Pig, and some other Ungulates, there is first of all a single
hillock or cusp, and that later the additional cones arise separately.
An apparent stumbling-block raised by these investigations is
that it is not always the protocone or its equivalent in the upper
jaw which arises first, as it obviously ought to do phylogenetically.
This, however, is not a final argument in either direction. We
know from plenty of examples that ontogenetic processes some-
times do not correspond in their order with phylogenetic changes.
Thus in the mammalian heart the ventricle divides before the
auricle; and of course, phylogenetically, the reverse ought to
occur, since a divided auricle precedes a divided ventricle. This
method of development has, moreover, been interpreted otherwise.
It has been held to signify that the complex teeth of mammals
are indeed derived from simple cones but by the fusion of a
number of those cones.
On the other hand there are the claims of the multituber-
cular theory of the origin of mammalian teeth to be considered.
The palaeontological evidence has been already, to some extent,
utilised. The occurrence of such teeth among the possible fore-
runners of mammals, and in some of the most primitive types of
Mammalia, has been referred to. Senor Anieghino dwells upon
the sextubercular condition of many primitive mammals even
belonging to the Eutheria. In a recent communication1 he attempts
to identify six tubercles in the molars of types belonging to a
1 "On the Primitive Type of the Plcxodont Molars of Mammals," Proc. Zcol.
Soc. 1899, p. 555.